A fixation concept for plastic parts often called “captive screws” are used in production assembly lines in order to reduce assembly time, guarantee quality and minimize the risk of losing screws. The screws are pre-mounted on the part into a compression delimiter, metal sleeve, and will remain until the screw is assembled. Usually the concept for captive screws needs a special screw which is more expensive than a standard screw.
One prior art solution is disclosed in CA 2263025 A1 where a fastening of an assembly part on a carrier part with at least one fastening screw which comprises a screw head with engaging surfaces for a tool, a screw shank, at least one radial projection on the screw shank at a distance from the screw head and a thread at least at the distance of the radial projection from the screw head, with a sleeve with an inner diameter which corresponds at least to the outer diameter of the radial projection and of the thread, which is a narrow location with an inner diameter smaller than the diameter of the radial projection but at least as large as the outer diameter of the screw shank between the screw head and the radial projection, and which at the upper end comprises a head flange and at the lower end a foot flange into which the fastening screw is inserted. The narrow location is snapped over the radial projection of the fastening screw and the screw head is allocated to the outer side of the head flange, with a flexible assembly ring whose inner diameter is smaller than the outer diameter of the head flange and foot flange and which is arranged between the head flange and foot flange on the sleeve, with a through-bore in the assembly part whose inner diameter is at least as large as the outer diameter of the foot flange, into which the sleeve is inserted. The lower side of the head flange is indirectly or directly supported on the upper edge region of the through-bore. However, the inner diameter of the through-bore is dimensioned such that the assembly ring is radially pressed together therein and by way of this clamped in and/or the through-bore comprises an undercut with an inner diameter so that the assembly ring under radial compression and subsequent expansion is pushed through the undercut and by which the assembly ring is retained in the through-bore, and with a threaded bore in the carrier part, into which is screwed the thread of the fastening screw and on whose edge region the foot flange of the sleeve is supported.
A further solution is disclosed in WO 2004/099632 A1. An arrangement is described made of a plastic part and a metal insert with a captive screw, wherein the metal insert can be introduced into a fixing hole of the plastic part in order to define fixing forces transmitted to the plastic part when the plastic part is screwed to another component. The continuous hole of the metal insert has an oval cross-diameter, whereby the largest diametrical dimension is located in the longitudinal direction of the plastic part, in order to compensate tolerances in a longitudinal direction. The continuous hole of the metal insert is provided with two opposite projections in the form of acclivities which keep the fixing screw captive in the plastic part during mounting.
US 2002/0106257 A1 discloses an assembly unit that includes at least one component and at least one screw including a shank having an outer diameter and at least partially including a thread. The head of the screw is designed and arranged to rotate the screw. A supporting surface of the screw faces the component. The component for each screw includes a through hole having a diameter. At least one bush is associated with the at least one screw, and it is designed and arranged to be insertable into the respective through hole. The bush is made of plastic material, and it is designed and arranged to be fixedly connected in the through hole resulting in elastic deformation of the bush. The inner diameter of the bush is slightly more than the outer diameter of the shank. The supporting surface is designed and arranged to operatively contact the component after assembly without contacting the bush to transmit an axial force. The bush, the screw and the component are designed and arranged to be captively connected.
Further, U.S. Pat. No. 6,309,156 B1 discloses a mounting unit that includes a mounting part having an activation side defining an opening leading to a throughbore. A fixing part is insertable into the throughbore and is fastenable to the mounting part, the fixing part including: a head which allows a fastening of the fixing part to the mounting part, a shaft adjoining the head on a top end thereof and being insertable with its bottom portion and with radial play into the throughbore, fixing projections disposed at the bottom portion of the shaft and extending beyond a cross section of the shaft in a plane perpendicular to the shaft longitudinal axis, the fixing projections being effective over an entire circumference of the shaft for fastening the fixing part to the mounting part, and an undercutting space disposed between the head and the fixing projections. A holding sleeve is adapted to be fastened to the mounting part and to surround the shaft and the fixing projections with radial play after the fixing part is inserted into the mounting part. The sleeve includes a holding collar at a top end thereof having a plurality of holding projections distributed across its circumference which allow the fixing part to pass there through unhindered during its insertion into the mounting part. The holding projections are deformable such that they project into the undercutting space in a radially inward direction after an insertion of the fixing part into the mounting part thereby lockingly opposing a loosening of the fixing part with respect to the mounting part.
U.S. Pat. No. 6,059,503 discloses a captivated fastener assembly and method of making it which incorporates a flexible retainer which is designed to allow the fastener to slide axially relative to the work piece while preventing separation thereof. The retainer eliminates the need for specially formed fasteners or work pieces.